In recent years, an energy saving trend on industrial products has been strong in view of environmental problem, rise of crude oil price, etc. The field of construction vehicles, work vehicles, etc. in which hydraulic drive systems using Diesel engines have heretofore played a central role is also on the same trend so that there has been an increasing number of instances using electrification to increase efficiency and improve energy saving.
When, for example, a drive portion of a vehicle in a construction machine etc. is electrified, that is, a power source thereof is set as an electric motor, lots of energy saving effects such as engine's high efficiency drive (hybrid), improvement in power transmission efficiency, recovery of regenerative electric power, etc. in addition to reduction of exhaust gas can be expected. Incidentally, in the aforementioned field of construction vehicles, work vehicles, etc., electrification of forklifts has been most advanced, and “electric-powered forklifts” in which motors are driven using electric power of batteries have taken the lead ahead of other vehicles and have been put into practical use. Consecutively to this, “hybrid vehicles” using combination of Diesel engines and electric motors in hydraulic excavators, engine type forklifts etc. have recently started being produced.
Among these construction machines and work vehicles on the trend toward ecology and energy saving using electrification, wheel loaders are vehicles which can be expected to have a comparatively large effect on reduction of fuel consumption when the vehicles are made hybrid. For example, as shown in FIG. 10, a background-art wheel loader is a work vehicle which has a travel portion (wheel portion) and a front hydraulic work portion (lift/bucket portion) so that while motive power of an engine 1 is transmitted to tires through a torque converter 2 and a transmission (T/M) 3 to make the vehicle travel, the bucket portion of the front hydraulic work device 5 using a hydraulic pump 4 as a drive source carries soil etc. In this manner, the wheel loader most frequently used currently uses only the engine as a power source and is driven to travel by the torque converter 2 and the transmission (T/M) 3 while the front hydraulic work portion is driven by the hydraulic pump 4.
When a travel drive portion of the aforementioned background-art wheel loader is electrified, transmission efficiency of the torque converter 2 and the transmission 3 can be improved up to transmission efficiency using electricity. Further, since the wheel loader repeats travel start and stop operations frequently during work, brake energy which has been heretofore emitted as heat loss of mechanical brake can be recovered and reused as regenerative electric power when the travel drive portion is electrified.
It is said that fuel consumption can be generally reduced by about a few tens of percent when part of the drive device of the current wheel loader is electrified and hybridized as described above. Some configurations are thought of for achievement of the hybridization. First, when vehicles including general cars are thought of, hybrid models are roughly classified into two types, i.e. a parallel hybrid type and a series hybrid type. Of them, the series hybrid type is a form in which a motive power transmission path between an engine and a motor is connected in series, that is, a generator is driven by motive power of the engine and an electric motor is driven by electric power generated by the generator. On the other hand, the parallel hybrid type is a form in which an engine and an electric storage device such as a battery or a large capacity capacitor are used together and motive power of the engine is mechanically assisted by motive power of an electric motor directly.
Various systems can be used as a system for transmitting the motive power of the engine assisted by the electric motor. For example, a system using combination of a torque converter and a transmission (T/M) or a system generally called HST using a hydraulic drive device can be used in the same manner as in the travel portion of the background-art wheel loader. In addition, a configuration in which a planetary gear and an electric motor are combined to perform electric gear change has also been recently proposed as a system which can most efficiently transmit the motive power of the engine mechanically. Thus, various system configurations have been conceived as the hybrid device. When a car or a construction machine is hybridized, it is necessary to prepare an optimal hybrid configuration in accordance with work/operation contents, device specifications, etc. of the car or construction machine.
In a work vehicle (a wheel loader, a dump truck, etc.) the invention is aimed at, an engine bears not only motive power required for travelling but also motive power requested by a front hydraulic work device and accounting for a comparatively large ratio. It is considered that, of those system configurations, in the aforementioned parallel hybrid system, an assist method using an electric motor may be comparatively complicated. It is therefore considered that the series hybrid type which is not so popular for general cars may be applied effectively.
A configuration example in the case where a general series hybrid system is applied to a wheel loader is shown in FIG. 11. The configuration example shown in FIG. 11 is a hybrid configuration in which a travel portion of a drive portion of the wheel loader is electrified and in which a motor/generator (M/G) 6 is mounted on an output shaft of an engine 1, an inverter 7 controls the motor/generator (M/G) 6, a drive motor 9 is attached to an output shaft (propeller shaft) 8 of the travel portion, and an inverter 10 controls the drive motor 9. An electric storage device 11 is electrically connected to the inverters 7 and 10 through a DC-DC converter 12 so that DC electric power can be supplied to and received from those electric power inverters. Particularly in the example of FIG. 11, the electric storage device 11 is described as an electric double layer capacitor. Buck-boost control of capacitor voltage is performed by the DC-DC converter 12 so as to receive/supply DC power from/to the inverters 7 and 10.
Similarly to the background-art wheel loader in FIG. 10, the hybrid wheel loader shown in FIG. 11 is also provided with a hydraulic pump 4 which supplies oil to a front hydraulic work device 5 for performing a work of excavating soil etc. in order to perform work in accordance with an object. On the other hand, the vehicle is driven to travel by the drive motor 9 using electric power generated by the motor/generator (M/G) 6 mainly based on motive power of the engine 1. Loss of a torque converter 2 is large in the background-art wheel loader in FIG. 10. However, when this portion is electrified as shown in FIG. 11, improvement in motive power transmission efficiency can be expected. During travelling, the electric storage device 11 can absorb regenerative electric power generated when the vehicle is braked, and assist power (torque) to the engine 1 to thereby contribute to reduction of the energy consumed by the vehicle.
When the wheel loader is the hybrid system shown in FIG. 11, the following problems can be conceived. An example of travel drive performance requested on the wheel loader is shown in FIG. 12. The wheel loader generally performs a work of carrying soil etc. and an excavation work of digging natural ground while the wheel loader travels with its four wheels. Accordingly, the wheel loader requires a large driving force at stoppage or extremely low speed. In addition, the wheel loader travels at high speed up to a vehicle speed of about 30 to 40 km/h. Thus, as shown in FIG. 12, the wheel loader is largely different from an operation range requested on a general industrial electric motor and is requested to have travel drive performance in a wide operation range. When it is intended to achieve such travel drive performance in the hybrid system in which the wheel loader can travel with a single drive motor 9 as shown in FIG. 11, both the driving with high torque at a low speed and the driving in a high speed rotation range have to be covered only by the drive motor 9. Accordingly, the electric motor is required to have a capacity about twice as large as the requested output. When the electric motor has a capacity about twice as large as the performance actually requested, it is conceived that it may be very difficult to mount the electric motor on the vehicle.
Therefore, in order to solve the aforementioned problem, it is an effective means that the electric motor for travel drive is split into plural pieces and the pieces are mounted. As a technique concerned with a work machine using two electric motors, for example, Patent Literature 1 has given disclosure about a travel work machine including a traveling body and a work device, characterized in that: the travel work machine further includes: a first electric motor; a second electric motor; and a torque distributing and transmitting mechanism into which drive torques of the first and second electric motors are inputted and which distributes and transmits the drive torques of the first and second electric motors to a drive shaft of the traveling body and a drive shaft of the work device. According to this, there is the following description. That is, the total of the capacities of the electric motors is reduced and configuration and control contents for allocating these drive powers of the electric motors to the respective drive shafts are made simple when the traveling body and the work device of the travel work machine are operated with the electric motors used as drive sources. In this manner, device cost can be reduced and energy efficiency can be improved.